limbo: encapsulate request inside write_de/promote

box_issue_promote/demote used to make WAL write and synchro
command apply in 2 separate synchro requests. Soon these APIs are
going to change a little. This patch moves the request creation
into the limbo internals so as box wouldn't care about request
structs.

Box now calls a single txn_limbo_write_promote/demote() function
which writes and applies a request under the hood.

Part of #6842

NO_DOC=Refactoring
NO_TEST=Refactoring
NO_CHANGELOG=Refactoring